Fixes #41457.

This changes all of our uses of `NETCOREAPP3_1` to `NETCOREAPP`. There
is no value in distinguishing the exact .NET Core version we use in our
source tree as we only target one version. Also going forward it's not
expected that our .NET Core specific code will be version specific. If
that arises we can go back to version specific defines for those cases.

This also removes the last references to `netcoreapp1.1`,
`netcoreapp2.1` and `netcoreapp3.0` from our code.

Fixes #41402

When performing analyzer execution from IDE for open files using [CompilationWithAnalyzers.GetAnalyzerSemanticDiagnosticsAsync](http://sourceroslyn.io/#q=CompilationWithAnalyzers.GetAnalyzerSemanticDiagnosticsAsync) API, we were not properly reseting the analyzer state after executing operation actions. This caused any subsequent operation block registrations by the same analyzer to be skipped in the analysis callbacks. This issue does not affect command line analyzer execution as we always run the analyzers on the entire compilation, and hence do not perform any partial state tracking.
Verified that the added unit test fails prior to the fix.

* MethodSymbol and FieldSymbol equality fixes:
 - Defer the equality of method/field symbol to the substituted symbol when compared
 - Make the hashcode of substituted method/field defer to the original definition

Rename from IncludeNonNullableReferenceTypeModifier to IncludeNotNullableReferenceTypeModifier (#41332)

This changes the C# compiler so that it multi-targets between
`netstandard2.0` and `netcoreapp3.1`.

Closes #41304

Decided to remove the property based approach to specifying a target
framework to just specifying `netcoreapp3.1` directly. The reason for
this is the following:

The advantage of the property is it makes it "easy" to change to a new
target framework in the future. That benefit is actually pretty minimal.
A simple find and replace operation is **extremely** effective in our
code base (it's less key strokes than this message). Hence the benefit
is minimal.

The downside of the property is that our code doesn't look like customer
code. Or rather it diverges from the practices that we publish. In
general I prefer to keep our code as standard as possible unless there
is a good reason to deviate. There just doesn't seem to be one here.

Change Roslyn to target netcoreapp3.1 when building .NET Core assets.

Previously the code targetted a mix of netcoreapp2.1 and netcoreapp3.0.
The mix is due to default interfaces only being supported on
netcoreapp3.0 and hence our testing needed to use that. Yet at the same
time we were required to ship the compiler in SDKS that targetted
netcoreapp2.1. Now we can universally target netcoreapp3.1.

Fixes #40629

See #41273

* Move "is telemetry allowed" flag to DiagnosticAnalyzerInfoCache

* Clean up DiagnosticService

* Fixes

* Remove analyzer exception telemetry reporting.

* Remove reporting analyzer exceptions from SupportedDiagnostics.

The diagnostics will be reported when the compiler runs the analyzer on the compilation.

* Remove unused parameter

* Clean up analyzer telemetry reporting.

* Test fixes

* Feedback

Fixes #41207.